The present invention relates to the field of pressure sensing and, more particularly, to a pressure gauge for high pressure water systems such as in fire fighting equipment.
Mechanical pressure measuring instruments such as pressure gauges have employed a sensing element a curved or twisted metallic tube which is flattened in cross section. The tube is closed at one end, and has a second end open to the fluid pressure to be sensed. As fluid pressure increases, the tube tends to become more circular in cross section and tends to straighten. The nineteenth-century French inventor, Eugene Bourdon, recognized that the motion of the closed end of such a tube in response to an applied pressure is proportional to the applied pressure and, thus, provides a measure of the pressure. The Bourdon tube still bears the inventor""s name in recognition of his inventive contribution.
In many prior art pressure gauges, the fluid whose pressure is to be sensed is allowed to enter the Bourdon tube to thereby apply pressure and cause movement of the tip of the tube. With gauges of this type, water often remains trapped in the closed end of the Bourdon tube when the high pressure water system is shut down and drained. Gauges with water trapped in this manner have a tendency to freeze and be damaged by low ambient temperatures, a common problem when fighting fires in cold winter climates.
In addition, those skilled in the art know that mechanical pressure sensors are affected by oscillations caused by a pulsating source of fluid pressure. That is, the indicator needle in the pressure gauge oscillates responsive to pressure pulses in the system, thereby making it difficult or impossible to obtain an accurate pressure reading.
One approach to addressing these problems is presented in U.S. Pat. No. 4,184,375 issued to Tommy L. Gray on Jan. 22, 1980, which describes a pressure gauge having a bladder, or diaphragm, isolating the pressure sensing apparatus from the fluid whose pressure is being sensed. The sensing apparatus, including the Bourdon tube, could therefore be filled with another fluid, preferably a low temperature resistant instrument oil. The oil filling the Bourdon tube produces a dampening effect which lessens indicator needle oscillation responsive to pressure pulses in the system.
Nevertheless, the fluid-filled Bourdon tube design still suffers from susceptibility to the well known xe2x80x9cwater hammerxe2x80x9d effect, a rapid spike in pressure generally caused by slamming a valve or nozzle closed during operation of a high pressure water system, thereby stopping large volumes of water almost instantaneously. Consequently, a pressure wave or fluid pressure pulse is generated which travels along the fluid in the system to all associated components, including the hose, supply pump, plumbing and associated water pipes, even the street water mains. Such a fluid pressure pulse is known to adversely affect pressure gauges by permanently distorting the Bourdon tube, thereby requiring replacement of the gauge.
Another approach to dampening, in addition to the use of a fluid-filled Bourdon tube, is to fill the entire pressure gauge housing with a fluid intended to dampen undesirable needle fluctuations. Predictably, such fluid-filled pressure gauges are more expensive to manufacture, and consequently must be sold at higher prices. Yet another method involves surrounding the needle pinion with a dampening agent so as to substantially absorb undesired vibrations, again resulting in increased manufacturing costs and leading to higher equipment prices.
With the foregoing in mind, the present invention advantageously provides a pressure sensing apparatus which is substantially resistant to pressure pulses in a high-pressure water system, and which provides suppression of unwanted indicator needle oscillations without the need for a fluid-filled gauge housing. In that regard, U.S. Pat. No. 4,184,375 is hereby incorporated by reference in its entirety in the description of the present invention. The present invention, therefore, provides an effective approach to substantially dampening undesirable indicator needle oscillations and damage to the apparatus caused by pressure pulses, while containing manufacturing costs for the instrument.
The pressure sensing apparatus of the present invention comprises a pressure sensitive element, preferably a Bourdon tube, a socket, a diaphragm, and a dampener. The Bourdon tube is connected to a source of fluid pressure for sensing pressure changes therein, and contains a pressure sensitive medium. The socket has a bore therethrough and is positioned to fluidly connect the Bourdon tube with the source of pressure. The diaphragm is positioned in the bore of the socket between the Bourdon tube and the source of pressure so as to isolate the pressure sensitive medium from the fluid whose pressure is sensed. A dampener is positioned between the diaphragm and the Bourdon tube, the dampener having a porous body effective for substantially dampening transmission of a pressure pulse from the source of pressure to the pressure sensitive medium.
The invention also includes a dampener adapted for positioning between the pressure sensitive element and the source of pressure being sensed so as to protect the pressure sensing apparatus from a pressure pulses. The dampener comprises a metallic body having a plurality of pores extending through the body along random paths, the plurality of pores allowing passage of fluid pressure therethrough while substantially dampening a fluid pressure pulse from the source of fluid pressure.
In a pressure sensing apparatus connected to a source of fluid pressure, yet another aspect of the invention includes a method of protecting the sensing apparatus from a pressure pulse traveling along the fluid. The method comprises forming a metallic body having a plurality of pores extending therethrough, positioning the metallic body relative to the source of fluid pressure so as to require fluid pressure to pass through the plurality of pores before being sensed by the pressure sensing apparatus, and passing the fluid pressure through the plurality of pores so as to substantially dampen the pressure pulse.